During the drilling of an oil or gas well successive measurements are made of various characteristics or properties of the earth formations being penetrated by the drill bit while it progressively excavates the borehole. Heretofore the majority of these measurements could not be made without discontinuing the drilling operation and removing the drill string and drill bit from the borehole long enough to enable typical wireline logging operations to be conducted in the open borehole. With the introduction of measuring-while-drilling or so-called "MWD" tools into commercial service, it has become feasible to successively measure various formation properties and characteristics as the borehole is being drilled and to transmit real-time signals representative of these measurements through the mud stream flowing in the drill string to appropriate detecting-and-recording equipment located at the surface.
Among the more-important open hole logging measurements are those characteristics of the earth formations which may be measured by techniques which utilize radiation. Inasmuch as measurements of the natural gamma radiation from the formations require only a gamma-ray detector and typical electronic circuits for controlling the MWD signaler, it has not been particularly difficult to make these measurements by the instrumentation in a MWD tool. Typical MWD tools that have this capability are shown, for example, in FIG. 4 of U.S. Pat. No. 3,255,353. On the other hand, as depicted in FIG. 1 of that patent, if other radioactivity characteristics of formations are to measured, the MWD tool must also carry a suitable radiation source such as a typical radioactive chemical source. Since the measurement of formation density is significantly influenced by borehole fluids, as described in U.S. Pat. No. 4,596,926 it has been proposed to compensate for the borehole fluids by arranging an array of radioactive sources and radiation detectors around the tool body.
There is, of course, always a risk that a MWD tool will become inadvertently stuck in the borehole during the course of a typical drilling operation. Should the MWD tool or drill string become seriously stuck, it may be necessary to remove as much of the drill string from the borehole as is possible and then employ appropriate "fishing" techniques to recover the remaining portion of the drill string as well as the MWD tool and the drill bit from the borehole before the drilling operation can be resumed. Such fishing operations may, however, impose such severe impacts on the MWD tool that its inner components could be seriously damaged before the tool can be recovered. Thus, should a MWD tool become stuck in a borehole, it is desirable to recover as much of the tool as is possible before starting the fishing procedures.
Whenever radioactive materials are used, it is essential to adopt procedures and design equipment that prevents the exposure of personnel on the rig floor during the routine installation and removal of sources used in the MWD tool. Additionally, whenever the source is arranged in a tool body immediately above the drill bit such as with the MWD tool depicted in U.S. Pat. No. 3,255,353, the removal and replacement of the bit could pose unnecessary exposure to rig personnel due to the proximity of the source unless special precautions are taken. With that arrangement, at least the lower portion of the MWD tool is suspended in the derrick while the drill bit is being uncoupled from the tool body carrying the source or while the radioactive source is being removed from the source chamber prior to the removal of the bit. This procedure must, of course, be reversed whenever the source is being installed into the MWD tool. Thus, whenever this prior-art tool is out of the borehole, it is quite difficult to protect the workers on the rig floor so long as the radioactive source is not safely enclosed in a shield. Accordingly, the periodic assembly and disassembly of the tool and the drill bit will subject the workers on the rig floor so long as the radioactive source is not safely enclosed in a shield. Accordingly, the periodic assembly and disassembly of the tool and the drill bit will subject the workers on the rig floor to exposures to radiation which are best avoided. Potential exposure is correspondingly increased whenever unexpected problems or delays in the assembly or disassembly of the MWD tool or drill bit occur or when an event occurs which requires some of the workers to work closer to the tool than would otherwise be necessary.
Even greater difficulties are presented with a MWD tool having a plurality of circumferential radioactive sources in the wall of a tool body such as shown in U.S. Pat. No. 4,596,926. For one thing, the radioactive sources in such a tool are mounted in lateral chambers that are closed by threaded port plugs. As a result, each time that the tool is removed from a borehole, the tool body carrying the sources will be caked with a thick layer of a gummy mudcake that must be scraped or washed off before the port plugs can be removed. With several sources to be removed, it will require a significant amount of time to locate the several port plugs, to disengage safety locking features designed to prevent inadvertent loss of the source while the borehole is being drilled, to remove the plugs and sources and to place the sources into suitable shielded source carriers. A similar amount of time will be required to reinstall the several sources into their respective source chambers when the MWD tool is being readied for service. Moreover, the continuous abrasion of the tool string against the borehole wall during a drilling operation may damage the external surfaces of the tool body around the entrance to a source chamber to such an extent that it may become difficult to remove the port plug. Should this occur, the workers will be compelled to remain close to the tool body for extended periods of time in order to remove the port plugs and the sources.